This invention relates to magnetrons.
Magnetrons typically use permanent magnets to set up a magnetic field through the interaction region. AlNiCo is often used as the magnetic material and is relatively easy to magnetise. As a result, it is found convenient to buy the material in a demagnetised state and to magnetise it in the finished magnetron. It is even possible to make fine adjustments to the magnetic field strength by controlled degaussing of the magnet using an alternating magnetic field generated by coils carrying an a.c. current.
Use of high energy magnetic materials such as samarium-cobalt or neodymium-iron-boron enables much smaller and lighter magnetrons to be realised but such magnetic material is much more difficult to magnetise and it is generally necessary to magnetise the material during manufacture, meaning that the magnets are bought in a fully magnetised state.
However, it may sometimes be necessary to trim the magnetic field in order that the magnetron will operate at the desired operating point of current and voltage.
Some existing methods of adjusting the magnetic field strength existing in a magnetron are described with reference to FIG. 1, which is a perspective view of a part of a known magnetron arrangement.
A magnetron is an evacuated device comprising a plurality of resonant cavities surrounding an interaction region where electrons emitted from a hot cathode are subjected to the combined effects of crossed electric and magnetic fields. The magnetic field is often focussed across the interaction region by means of high permeability pole-pieces, which sometimes form part of the vacuum envelope. Detail of the magnetron is omitted from FIG. 1 but the interaction region is positioned between pole pieces 1, 2 of a permanent magnet.
The magnetic field can be generated by a horseshoe magnet or by a pair of magnets with a magnetically permeable return path. The field can be applied directly without pole pieces but more commonly the field is concentrated by means of high permeability pole-pieces. The pole pieces may be in intimate contact with the magnet(s) or they may connect via an intermediate pole-shoe for convenience in construction. FIG. 1 shows an example where the field is provided by magnet blocks 3, 4 of one polarity and magnet blocks 5, 6 of the opposite polarity. The blocks 7, 8 are pole shoes for housing the respective pole pieces 1, 2. Additional pairs of magnet blocks on the far side of pole shoes 7, 8 symmetrical with the magnetic blocks 3-6 may also be provided. Thin sheets of mild steel 9, 10 provide the magnetic return path.
One known method of adjusting the strength of the magnetic field through the magnetron is by the use of corner shunts, such as that illustrated by the reference numerals 11, 12. These corner shunts are of mild steel, and some of the magnetic flux is diverted through them. This reduces the magnetic field available to extend through the magnetron itself. They can be used where it is desired to reduce the magnetic field strength in the working gap between the pole pieces 1, 2.
Alternatively, flat shunts, consisting of one arm only of the illustrated corner shunts, may be employed to reduce the magnetic field in the working gap.
Another known method of achieving this objective is to provide additional sheets of thin mild steel for the magnetic return path.
It has been proposed (U.S. Pat. No. 4,338,545) to adjust the magnetic field in the interaction space to compensate for changes in field strength resulting from temperature variation by automatic displacement of auxiliary pole pieces in response to deformation of a bimetallic member.
It has also been proposed (UK Patent No. 826 822) to displace a magnetic shunt between the pole pieces and pole shoes of a magnetron in a radial direction towards the axis of the anode in order to considerably reduce the magnetic forces to assist in the magnetron being assembled/disassembled.